1. Field of the Invention
The present invention relates to intense relativistic electron beams, and more particularly to an improved cold-cathode electron gun that does not require an evacuated envelope.
2. Description of the Prior Art
Relativistic electron beams have a variety of commercial, scientific and military applications. High power electron guns provide a source of particulate radiation for industrial radiography, chemical polymerization, sterilization of food and drugs, and directed energy concepts. Prior art methods of generating high power electron beams have been described in Winterberg, F. "The Potential of Electric Cloud Modification by Intense Relativistic Electron Beams", Zeitschift fuer Meteorolgie, 25, No. 3,180 (1975). A self-accelerated cold-cathode electron gun described therein uses an evacuated dielectric tube containing the cathode emitter spaced from a grounded titanium foil that seals the tube end. The gun is excited as the load of a Marx electric surge generator. During the application of a high voltage surge, electrons are released from the cathode material by cold-field emission, due to the high electric field at the cathode face. The electrons accelerate toward the foil window and, if their energy is sufficiently high, penetrate into the open air.
When this type of gun is used to produce high power beams, the foil window is vaporized by energy deposition and the gun is destroyed. The presence of the grounded foil also limits the voltage that can be applied to the cathode without flashover occurring to the grounded window. Additionally, the closed evacuated system must be provided with means to limit the hardening of the gun's vacuum due to the interaction of the residual gas and electron beam. The hardening produces unwanted variations in the gun's electrical impedance.
Winterberg also describes a workpiece-accelerated cold-cathode electron gun that does not require a physical window but develops the beam in a vacuum and directs the beam into the air via a fractional vacuum gate. It is necessary to pump the gate and provide auxiliary magnetic fields to guide the beam through the gate.
The gun designs discussed by Winterberg suffer from an accumulation of heat at the cathode electrode and dielectric gun barrel, which if not removed leads to wear and destruction of the gun.
The projection of intense relativistic electron beams into the free atmosphere requires a gun that can support very high power levels and be fired at a high repetitive rate. The gun should be free from physical windows, auxiliary magnetic fields, drive voltage limitations due to cathode-anode spacing, self-destruction due to heating, and vacuum hardening.